This invention relates to the purification of natural gas, and, more particularly, to the removal of nitrogen from natural gas by use of a molecular sieve in a novel pressure swing adsorption (PSA) process.
The removal of nitrogen from natural gas is of considerable importance inasmuch as nitrogen is present to a significant extent. Nitrogen contamination lowers the heating value of the natural gas and increases the transportation cost based on unit heating value.
Applications aimed at removing nitrogen and other impurities from natural gas streams provide significant benefits to the U.S. economy. In 1993, the Gas Research Institute (GRI) estimated that 10-15% (xe2x88x9222 trillion cubic feet) of the natural gas reserves in the U.S. are defined as sub-quality due to contamination with nitrogen, carbon dioxide, and sulfur. Most of these reserves, however, have discounted market potential, if they are marketable at all, due to the inability to cost effectively remove the nitrogen. Nitrogen and carbon dioxide are inert gases with no BTU value and must be removed to low levels ( less than 4% typically) before the gas can be sold.
Concurrently, the U.S. has proven reserves of natural gas totaling 167 trillion cubic feet. Over the past five years, annual consumption has exceeded the amount of new reserves that were found. This trend could result in higher cost natural gas and possible supply shortages in the future. As the U.S. reserves are produced and depleted, finding new, clean gas reserves involves more costly exploration efforts. This usually involves off shore exploration and/or deeper drilling onshore, both of which are expensive. Moreover, unlike crude oil, it is not economical to bring imports of natural gas into North America, therefore pricing of natural gas could be expected to rise forcing end users to seek alternative fuels, such as oil and coal, that are not as clean burning as gas. While base consumption for natural gas in the U.S. is projected to grow at 2-3% annually for the next ten years, one segment may grow much more rapidly. Natural gas usage in electric power generation is expected to grow rapidly because natural gas is efficient and cleaner burning allowing utilities to reduce emissions. Accordingly, there is a need to develop a cost-effective method of upgrading currently unmarketable sub-quality reserves in the U.S. thereby increasing the proven reserve inventory.
Methods heretofore known for purification of natural gas, in particular, nitrogen removal, may be divided roughly into three classifications:
(a) Methods involving fractional distillation at low temperature and (usually) high pressure, i.e. cryogenics. Since nitrogen has a lower boiling point than methane and the other hydrocarbons present in natural gas, it may be removed as a gas on liquefying the remaining constituents, which are then revaporized.
(b) By selective adsorption of the methane and higher hydrocarbons on an adsorbent such as activated charcoal. The adsorbed gases are then desorbed to give a gas free of nitrogen.
(c) Miscellaneous processes involving selective diffusion through a series of organic membranes, formation of lithium nitride by treatment with lithium amalgam, absorption of the nitrogen in liquid ammonia or in liquid sulphur dioxide.
The principal disadvantage of the fractional distillation and adsorption processes is that they remove the major component, methane, from the minor component, nitrogen, instead of the reverse. In cryogenic processing, almost the entire volume of natural gas must be refrigerated, usually compressed, and then heated again. Accordingly, cryogenic processing is expensive to install and operate, limiting its application to a small segment of reserves. Cryogenic technology is believed only capable of cost effectively purifying reserves, which exceed 50,000,000 standard cubic feet of gas per day and as well having nitrogen contamination level of 15% or more. Gas reserves that do not fit these criteria are not currently being purified. The potential value of this gas is totally lost as the wells are usually capped. The processes suggested under paragraph (c) above are handicapped by an unsatisfactory degree of separation or by the use of very expensive materials.
In smaller-scale natural gas operations as well as in other areas such as synthesis gas and coke oven gas processing, adsorption processes can be especially well suited. The adsorption capacities of adsorption units can, in many cases, be readily adapted to process gas mixtures of varying nitrogen content without equipment modifications, i.e. by adjusting adsorption cycle times. Moreover, adsorption units can be conveniently skid-mounted, thus providing easy mobility between gas processing locations. Further, adsorption processes are often desirable because more than one component can be removed from the gas. As noted above, nitrogen-containing gases often contain other gases that contain molecules having smaller molecular dimensions than nitrogen, e.g., for natural gas, carbon dioxide, oxygen and water, and for coke oven gas, carbon monoxide.
U.S. Pat. No. 2,843,219 discloses a process for removing nitrogen from natural gas utilizing zeolites broadly and contains specific examples for the use of zeolite 4A. The process disclosed in the patent suggests use of a first nitrogen selective adsorbent zeolite in combination with a second methane selective adsorbent. The molecular sieve adsorbent for removing nitrogen is primarily regenerated during desorption by thermal swing. A moving bed adsorption/desorption process is necessary for providing sufficient heat for the thermal swing desorption. The moving bed process specifically disclosed in this patent is not practical and it does not provide a cost efficient method for the separation of nitrogen from natural gas in view of high equipment and maintenance costs and degradation of the adsorbent by attrition due to contact with the moving adsorbent portables.
Despite the advantageous aspects of adsorption processes, the adsorptive separation of nitrogen from methane has been found to be particularly difficult. The primary problem is in finding an adsorbent that has sufficient selectivity for nitrogen over methane in order to provide a commercially viable process. In general, selectivity is related to polarizability, and methane is more polarizable than nitrogen. Therefore, the equilibrium adsorption selectivity of essentially all known adsorbents such as large pore zeolites, carbon, silica gel, alumina, etc. all favor methane over nitrogen. However, since nitrogen is a smaller molecule than methane, it is possible to have a small pore zeolite, which adsorbs nitrogen faster than methane. Clinoptilolite is one of the zeolites, which has been disclosed in literature as a rate selective adsorbent for the separation of nitrogen and methane.
U.S. Pat. No. 4,964,889 discloses the use of natural zeolites such as clinoptilolites having a magnesium cation content of at least 5 equivalent percent of the ion-exchangeable cations in the clinoptilolite molecular sieve for the removal of nitrogen from natural gas. The patent discloses that the separation can be performed by any known adsorption cycle such as pressure swing, thermal swing, displacement purge or nonadsorbable purge, although pressure swing adsorption is preferred. However, this patent is silent as to specifics of the process, such as steps for treating the waste gas, nor is there disclosure of a high overall system recovery.
In general, first applications of PSA processes were performed to achieve the objective of removing smaller quantities of adsorbable components from essentially non-adsorbable gases. Examples of such processes are the removal of water from air, also called heatless drying, or the removal of smaller quantities of impurities from hydrogen. Later this technology was extended to bulk separations such as the recovery of pure hydrogen from a stream containing 30 to 90 mole percent of hydrogen and other readily adsorbable components like carbon monoxide or dioxide, or, for example, the recovery of oxygen from air by selectively adsorbing nitrogen onto molecular sieves.
The carrying out of the PSA processes in multi-bed systems is illustrated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to a system having at least four beds. As is generally known and described in this patent, the PSA process is commonly performed in a cycle of a processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) co-current depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower pressure; (4) purge; and (5) pressurization. The void space gas released during the co-current depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure.
Similar systems are known which utilize three beds for separations. See, for example, U.S. Pat. No. 3,738,087 to McCombs. The faster the beds perform steps 1 to 5 to complete a cycle, the smaller the beds can be when used to handle a given hourly feed gas flow. If two steps are performed simultaneously, the number of beds can be reduced or the speed of cycling increased; thus, reduced costs are obtainable.
U.S. Pat. No. 4,589,888 to Hiscock, et. al. discloses that reduced cycle times are achieved by an advantageous combination of specific simultaneous processing steps. The gas released upon co-current depressurization from higher adsorption pressure is employed simultaneously for pressure equalization and purge purposes. Co-current depressurization is also performed at an intermediate pressure level, while countercurrent depressurization is simultaneously performed at the opposite end of the bed being depressurized.
U.S. Pat. No. 4,512,780 to Fuderer discloses a pressure swing adsorption process with intermediate product recovery. Three products are recovered from a pressure swing adsorption process utilizing a displacement step in conjunction with pressure equalization between beds of a multi-bed adsorption system. This process is not cost efficient for the recovery of two products.
Although pressure swing separation adsorption (PSA) has been used to separate a wide variety of gases, the simple fact remains that there is no commercially practiced PSA process for the separation of nitrogen from methane. This is due to many factors including the lack of a nitrogen specific adsorbent and environmental regulations.
As previously pointed out, a significant percentage of U.S. natural gas reserves contain more than 4% nitrogen. The bulk of these reserves cannot be exploited because no economical technology exists for removing nitrogen especially at low flow rates, i.e., less than 25 MMSCFD process feed gas. Cryogenic distillation is the only process being used to date on any scale to remove nitrogen from methane in natural gas. Cryogenic plants are not used more widely because they are expensive and complicated and exhibit poor scale down economics.
It is the primary objective of this invention to provide a commercially viable PSA process for removing nitrogen from natural gas.
A further object of the invention is to provide a PSA process for removing nitrogen from natural gas, which can provide a highly concentrated methane product at high process efficiencies.
Another object of this invention is to separate nitrogen from natural gas by PSA and yield a methane product of high heat value.
Still another object of the invention is to provide and maintain peak efficiency of the nitrogen-selective adsorbent during PSA separation of nitrogen from natural gas.
This invention provides a novel PSA system to remove nitrogen from natural gas. The PSA processes of this invention to remove nitrogen from natural gas also achieves high system hydrocarbon recovery and usage of recovered hydrocarbons from the feed gas to provide additional heat value to the methane product stream. In accordance with this invention, a natural gas feed is first passed through an adsorbent selective for C3+ hydrocarbon components operating in a PSA cycle and directing the product gas leaving the first PSA system to a second PSA system containing an adsorbent selective for nitrogen. The methane-rich product stream of the second PSA is used to purge heavy hydrocarbons off the first PSA adsorbent. The heavy hydrocarbons add heat value to the methane product.
In another aspect of this invention, increased efficiency of the two-stage PSA process described above is provided by recycling a co-current intermediate pressure dump stream to the second stage PSA feed.
The PSA process for removing nitrogen from methane is also maintained to provide a high purity methane product by periodically heating an off-line nitrogen-selective adsorbent bed with methane product to drive off co-adsorbed methane and increase nitrogen adsorbent capacity.